The project sponsor holds the ultimate accountability for the project's success and is typically responsible for approving significant decisions, including risk mitigation responses. Their role involves ensuring that the project aligns with business objectives and that risks are managed appropriately to achieve desired outcomes.
Reference:ISACA CRISC Review Manual, 7th Edition, Chapter 1: Governance, Section: Roles and Responsibilities.

The effectiveness of a control action plan's implementation can be measured by the extent to which it achieves the desired risk reduction. A control action plan is a set of actions that are designed to address the root causes of a risk and mitigate its impact or likelihood. The best indicator of the effectiveness of a control action plan's implementation is the reduced risk level, which means that the risk is either eliminated or brought within the acceptable range. The other options are not the best indicators, because they do not directly reflect the risk reduction. Increased number of controls may not necessarily reduce the risk level, especially if the controls are not aligned with the risk causes, objectives, and priorities. Increased risk appetite may indicate a higher tolerance for risk, but it does not mean that the risk level has been reduced. Stakeholder commitment may facilitate the implementation of the control action plan, but it does not guarantee the effectiveness of the plan. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 3: Risk Response, Section 3.2: Control Action Plan, p. 170-171.

Risk exposure is the product of risk likelihood and risk impact ratings. It represents the potential loss or damage that may result from a risk event. After implementing countermeasures, the risk likelihood and/or impact ratings may change, depending on the effectiveness of the countermeasures. Therefore, the risk exposure must also change to reflect the updated risk ratings. The other components of the register, such as risk owner, risk impact rating, and risk likelihood rating, may or may not change depending on the nature and scope of the countermeasures. References = Risk and Information Systems Control Study Manual, Chapter 2:
IT Risk Assessment, Section 2.4: IT Risk Response, page 87.

According to the CRISC Review Manual, the primary objective of providing an aggregated view of IT risk to business management is to enable consistent data on risk to be obtained, because it helps to ensure that the risk information is comparable, reliable, and accurate across the organization. An aggregated view of IT risk is a consolidated and comprehensive representation of the IT risk exposure and impact at the enterprise level, based on the risk identification, analysis, and evaluation processes. Providing an aggregated view of IT risk to business management allows them to understand the overall IT risk profile and performance, and to make informed decisions about the risk management strategies and priorities. The other options are not the primary objective of providing an aggregated view of IT risk, as they are related to other benefits or outcomes of the risk aggregation process. Allowing for proper review of risk tolerance is the objective of establishing the risk context, which defines the scope and boundaries of the risk management activities. Identifying dependencies for reporting risk is the outcome of the risk aggregation process, as it provides a clear and consistent structure and format for the risk communication and reporting. Providing consistent and clear terminology is the objective of developing the risk taxonomy, which is the system of classification and categorization of risks based on common characteristics and attributes. References = CRISC Review Manual, 7th Edition, Chapter 2, Section 2.1.2, page 69.